1. Field of the Invention
The present invention relates to an adapter power supply with reduced power consumption in a standby period.
2. Description of the Related Art
Adapter power supplies are widely used for obtaining a specified DC output voltage receiving a worldwide input from a commercial AC power source at a voltage of 90 to 264 Vac. The adapter power supplies are used for external power supplies to supply DC voltage to electronic devices such as small-sized television sets, printers, and personal computers of a notebook type. Such an adapter power supply has a basic construction as shown in FIG. 6 and comprises: a rectifying circuit DB for rectifying an AC voltage, an isolation transformer T connected to the rectifying circuit DB and having a primary winding T1 and a secondary winding T2, a switching element Q connected in series to the primary winding T1, and a control circuit CONT that is a control IC that performs self-oscillation and switching driving the switching element Q at a specified period. The adapter power supply further comprises in the secondary side, a diode D for rectifying a voltage generated on the secondary winding T2 of the isolation transformer T and an output capacitor Cout for smoothing the rectified voltage to deliver a DC output voltage Vout. Patent Document 1 (identified below), for example, discloses such an adapter power supply.
The information of the DC output voltage Vout is detected by a voltage detecting circuit Vsens provided in the secondary side of the isolation transformer T and is fed back through a photo-coupler PC. The information is specifically an error voltage of the DC output voltage Vout with respect to a reference voltage. The control circuit CONT that is a control IC controls the ON width of the switching element Q corresponding to the error voltage to obtain a constant DC output voltage Vout. The control circuit CONT operates with a driving power source of the voltage that is generated by the switching operation of the switching element Q and appears across the auxiliary winding T3 of the isolation transformer T.
Conventional technologies take measures to reduce power consumption during the standby period, in which the DC output voltage of an adapter power supply is changed over, for example, from a rated output voltage of 32 V to a standby period output voltage of 12 V in a period of no load or light load. Specifically, as shown in FIG. 7, a control signal showing an operating or heavy load condition is given from the electronic device to which the adapter power supply concerned supplies DC power, and the control signal turns ON a switch S to deliver a rated output voltage. When the control signal is interrupted in a no load or light load condition, the switch S is turned OFF.
The ON state of the switch S connects the auxiliary resistor R3 to the resistor R2 for output voltage detection in parallel to set a high level of voltage detection condition of the voltage detecting circuit Vsens, thereby setting the DC output voltage Vout at the rating output of 32 V. On the other hand, the OFF state of the switch S disconnects the auxiliary resistor R3 to set a low level of voltage detection condition of the voltage detecting circuit Vsens, thereby setting the DC output voltage Vout at the standby period output voltage of 12 V.
[Patent Document 1]
Japanese Unexamined Patent Application Publication No. 2011-015570
As described above, the power consumption in the adapter power supply is reliably reduced by decreasing the DC output voltage Vout of the adapter power supply under the no load or light load condition. Such control, however, needs to obtain a control signal from the side of the electronic device. In addition, a two-core power cable must be replaced by a three-core power cable including a signal transmission line. Moreover, the electronic device side has to be provided with an interface and a control program to give the control signal to the adapter power supply. Thus, the system construction as a whole becomes complicated.